The proposed research tests the assumption that human performance in domains such as speech, music, and typing involves the construction of mental representations. A cognitive theory of human performance must combine a) mental structures and units of knowledge with b) cognitive retrieval processes for their organization and implementation in performance. Theories of performance can be tested through study of production errors: performance breakdowns resulting in unintended output. A successful theory will specify the kinds of errors likely to occur, constraints on the forms that errors take, and the conditions that precipitate them. Musical behavior provides a rich domain in which to study cognitive processes underlying performance. With specialized equipment, methodology, and computer software, we can now precisely measure, analyze,and reproduce music performance. Experiments are described that manipulate the type and amount of information available prior to performance, affecting the mental structures and processes used to plan performance. The distribution and context of production errors provides evidence to test: a) the existence of mental plans prior to performance, b) the mental structures and processes from which performance is organized, and c) interdependencies among mental structures and processes. Additional experiments address changes in cognitive processes accompanying acquisition of skilled performance, and whether or not knowledge of performance plans affects music perception. The mental structures and processes underlying music performance may be based on cognitive principles governing other human performance systems as well.